This isn't going to earn me points, but...
I'm actually a ghostwriter who has written best selling books.
It takes more than good writing to be a best seller (Spoiler alert?) so that doesn't say much. My own book suffers from my inability to market. (I'm trying to learn, I swear!)
The biggest hurdle to "proving" you're a best seller is the dreaded NDA. Almost every book I've written has been under an NDA. (I write a 30k book a week, give or take, when my health allows. So at my lowest (2 books a month) to my highest (4 books a month) over the course of two years... well, I've written a lot of books. I've had clients give me bonuses (tips) when the books have hit best seller status. I doubt they'd lie to me about this. But by calling myself a best selling ghost writer, I've got an unsubstantiated claim that I *can't* prove without opening myself to legal troubles, not to mention getting my ghostwriting shut down in a blink.
That said, I'd NEVER publicly (or privately!) speak to anyone the way that poster spoke to the whole thread he was posting in. It's a disgrace to all ghostwriters. Questions are to be expected and are how we form good working partnerships with our respective clients. I'd never dream of speaking to anyone the way he did. It was unprofessional and frankly disgusting.
I do have to say, 0.025 per word is low for someone claiming best selling writing. It makes me nervous - I've heard a lot of clients complain that they've paid low rates and been given work that tiptoes plagiarism or even blatantly rips off best selling books by only changing locations, names, and other minute details. Most of them say they'd never go the cheap route again. I'm not saying that's what he's doing or that only low raters cheat, I'm just saying that
generally you get what you pay for. /shrug
I also wonder why he's on kboards instead of sites for freelancers like Upwork. In my experience places like kboards are not the... friendliest... to ghosts.
If anyone has questions about how ghosting works or anything like that, I'm happy to share. And I won't bite your head off or call you unprofessional. I've got thick skin. Fire away.

ETA: This is not a plug - I'm not offering services at this time.
(Received a PM to start a Ghosting thread - you can find it by going to the Publisher's Office > The 'How Ghosting Works' thread!)